Chicago, IL

Weather Forecast Office

September 7, 2021: Swaths of Large Hail and Damaging Winds Across Northern Illinois

Overview

Fast Facts:

Numerous severe thunderstorms tracked across northern Illinois and parts of northwest Indiana on Tuesday, September 7.

Significant hail larger than golf balls fell in parts of Lee and LaSalle Counties, including near and in the towns of West Brooklyn, Compton, Paw Paw, and Earlville, with some of the hail not melting until several hours after it fell.

September 7 marks the first day in 2021 during which hail at least 2" in diameter was reported in the NWS Chicago forecast area and is, so far, the day with the most hail reports in 2021. Typically, "big hail days" in our area occur during the spring months.

Swaths of wind damage to mainly utility poles and trees (including some trees falling onto structures) occurred, particularly from the Lisbon and Morris areas into the Minooka, Channahon, and Manhattan areas across Kendall, Grundy, and Will Counties. An NWS storm damage survey team assessed that this damage was caused by 60-85 mph winds, with no conclusive evidence of any tornadoes.

There was also significant wind damage in Gibson City, IL in Ford County.

The potential for severe weather on Tuesday was noted already on Sunday, with increasing potential noted later Monday into Tuesday morning. A Severe Thunderstorm Watch was issued shortly after 1 PM in the afternoon with numerous Warnings issued through the event.

The first NWS Chicago "Significant Severe" Thunderstorm Warning with WEA Activation was issued for baseball size hail in Earlville and northeast LaSalle County. The "Significant Severe" option for Severe Thunderstorms Warnings had been implemented in the summer of 2021.

Recap of Severe and Flooding Reports.

MRMS Radar Reflectivity at Lowest Altitude (RALA) from 11:00 AM to 9:00 PM CDT, September 7, 2021

Storm Reports
Minooka to Channahon Damage
Photos
Environment
Additional Info

Preliminary Storm Reports

September 7th Preliminary Storm Reports

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE CHICAGO IL
938 PM CDT THU SEP 9 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1232 PM     TSTM WND DMG     2 NW SHIRLAND           42.46N 89.22W
09/07/2021                   WINNEBAGO          IL   PUBLIC

            PICTURE OF TREE DOWN SENT VIA SOCIAL MEDIA.
            TIME IS ESTIMATED FROM RADAR.

1236 PM     HAIL             ROCKTON                 42.45N 89.07W
09/07/2021  E0.75 INCH       WINNEBAGO          IL   TRAINED SPOTTER

            CORRECTS TIME OF PREVIOUS HAIL REPORT FROM
            ROCKTON.

1255 PM     HAIL             1 NNW TIMBERLANE        42.35N 88.87W
09/07/2021  E1.75 INCH       BOONE              IL   TRAINED SPOTTER

            TIME ESTIMATED FROM RADAR.

1259 PM     HAIL             HARVARD                 42.42N 88.62W
09/07/2021  E1.50 INCH       MCHENRY            IL   PUBLIC

            TIME ESTIMATED BY RADAR.

0113 PM     TSTM WND DMG     WONDER LAKE             42.39N 88.35W
09/07/2021                   MCHENRY            IL   PUBLIC

            SMALL TREE ABOUT 6 INCHES IN DIAMETER
            SNAPPED. TIME ESTIMATED FROM RADAR.

0113 PM     HAIL             2 NNE NELSON            41.82N 89.60W
09/07/2021  U1.00 INCH       LEE                IL   PUBLIC

            QUARTER SIZE HAIL REPORT FROM MPING. TIME
            ESTIMATED FROM RADAR.

0116 PM     HAIL             2 E RICHMOND            42.48N 88.26W
09/07/2021  E1.00 INCH       MCHENRY            IL   TRAINED SPOTTER



0120 PM     HAIL             DIXON                   41.85N 89.48W
09/07/2021  E2.00 INCH       LEE                IL   PUBLIC

            TIME ESTIMATED FROM RADAR.

0120 PM     HAIL             SPRING GROVE            42.44N 88.24W
09/07/2021  E1.00 INCH       MCHENRY            IL   PUBLIC



0123 PM     HAIL             1 NW JOHNSBURG          42.39N 88.25W
09/07/2021  U1.00 INCH       MCHENRY            IL   PUBLIC

            QUARTER SIZE HAIL REPORT FROM MPING. TIME
            ESTIMATED FROM RADAR.

0126 PM     HAIL             1 ESE FOX LAKE          42.39N 88.16W
09/07/2021  E0.70 INCH       LAKE               IL   TRAINED SPOTTER



0127 PM     HAIL             WOODSTOCK               42.32N 88.45W
09/07/2021  E1.50 INCH       MCHENRY            IL   TRAINED SPOTTER



0129 PM     HAIL             ROUND LAKE              42.35N 88.09W
09/07/2021  E1.00 INCH       LAKE               IL   PUBLIC

            TIME ESTIMATED BY RADAR. LOCATION
            APPROXIMATED BY WFO BGM, CALLER ONLY GAVE
            ZIP CODE.

0129 PM     TSTM WND GST     2 SE BULL VALLEY        42.29N 88.32W
09/07/2021  M68 MPH          MCHENRY            IL   TRAINED SPOTTER

            MEASURED ON PERSONAL WEATHER STATION.

0130 PM     TSTM WND DMG     2 SSW PRAIRIE GROVE     42.26N 88.28W
09/07/2021                   MCHENRY            IL   TRAINED SPOTTER

            NUMEROUS TREE BRANCHES DOWNED AND SNAPPED,
            LARGE BRANCH BLOCKING WESTBOUND LANE OF
            IL-176. TIME ESTIMATED FROM RADAR.

0130 PM     HAIL             ROUND LAKE              42.36N 88.09W
09/07/2021  E1.00 INCH       LAKE               IL   PUBLIC

            TIME ESTIMATED FROM RADAR.

0135 PM     HAIL             4 N AMBOY               41.77N 89.33W
09/07/2021  E1.50 INCH       LEE                IL   PUBLIC

            REPORT RELAYED BY WTVO/WQRF. TIME AND
            LOCATION ESTIMATED FROM RADAR.

0136 PM     TSTM WND DMG     1 SSW ISLAND LAKE       42.26N 88.20W
09/07/2021                   MCHENRY            IL   PUBLIC

            MULTIPLE LARGE BRANCHES AND TREES DOWN ALONG
            ROBERTS RD. TIME ESTIMATED FROM RADAR.

0140 PM     HAIL             1 NE GAGES LAKE         42.36N 87.96W
09/07/2021  E0.88 INCH       LAKE               IL   TRAINED SPOTTER



0142 PM     TSTM WND DMG     LIBERTYVILLE            42.28N 87.95W
09/07/2021                   LAKE               IL   UNKNOWN

            REPORT RELAYED VIA NWS CHAT OF BRANCHES
            DOWN, UNKNOWN SIZE. TIME ESTIMATED BY RADAR.

0145 PM     TSTM WND DMG     VERNON HILLS            42.23N 87.96W
09/07/2021                   LAKE               IL   PUBLIC

            14 INCH DIAMETER TREE SNAPPED AT BASE. TIME
            ESTIMATED BY RADAR.

0148 PM     HAIL             3 NNW PAW PAW           41.73N 89.01W
09/07/2021  E1.00 INCH       LEE                IL   BROADCAST MEDIA



0148 PM     HEAVY RAIN       2 SE BULL VALLEY        42.29N 88.32W
09/07/2021  M1.10 INCH       MCHENRY            IL   TRAINED SPOTTER

            RAINFALL MEASURED ON PERSONAL WEATHER
            STATION OVER A 45 MINUTE PERIOD.

0152 PM     TSTM WND GST     2 SE WHEELING           42.12N 87.90W
09/07/2021  M58 MPH          COOK               IL   ASOS

            PWK ASOS PEAK GUST.

0154 PM     TSTM WND DMG     1 SE ISLAND LAKE        42.26N 88.18W
09/07/2021                   LAKE               IL   PUBLIC

            PICTURE OF LARGE TREE DOWN AND BLOCKING
            DARREL RD SOUTH OF IL-176 RELAYED VIA SOCIAL
            MEDIA.

0155 PM     TSTM WND DMG     DEERFIELD               42.17N 87.85W
09/07/2021                   LAKE               IL   PUBLIC

            VIDEO SHARED ON SOCIAL MEDIA SHOWS PART OF A
            LARGE TREE GETTING SNAPPED OFF AT ITS TOP.
            TIME ESTIMATED FROM RADAR.

0155 PM     TSTM WND DMG     1 NNE NORTHBROOK        42.14N 87.83W
09/07/2021                   COOK               IL   TRAINED SPOTTER

            SPOTTER REPORTS MULTIPLE 3 TO 4 INCH
            BRANCHES DOWN.

0158 PM     HAIL             WEST BROOKLYN           41.69N 89.15W
09/07/2021  E2.00 INCH       LEE                IL   PUBLIC

            PHOTO RECEIVED OF HAIL. ALSO REPORTED SMALL
            DENTS ON VEHICLES FROM THE HAIL IN TOWN.

0158 PM     HAIL             COMPTON                 41.69N 89.09W
09/07/2021  E1.00 INCH       LEE                IL   PUBLIC

            TIME ESTIMATED FROM RADAR. IMAGE WAS SENT IN
            VIA SOCIAL MEDIA.

0159 PM     TSTM WND DMG     HIGHLAND PARK           42.18N 87.80W
09/07/2021                   LAKE               IL   PUBLIC

            PICTURE OF A TREE DOWN NEAR HIGHLAND PARK
            RELAYED VIA SOCIAL MEDIA.

0201 PM     HAIL             COMPTON                 41.69N 89.09W
09/07/2021  E2.00 INCH       LEE                IL   PUBLIC

            UPDATES PREVIOUS HAIL REPORT FROM COMPTON.
            HAIL WAS AT A MINIMUM 2 INCHES IN DIAMETER.
            AT LEAST SIX AUTO WINDSHIELDS SMASHED. ONE
            RESIDENCE HAD MULTIPLE SKYLIGHTS SMASHED.
            TIME ESTIMATED FROM RADAR.

0203 PM     HAIL             2 SE COMPTON            41.68N 89.06W
09/07/2021  E1.75 INCH       LEE                IL   PUBLIC

            HAIL DAMAGE TO SIDING AT FARMSTEAD. TIME
            ESTIMATED BY RADAR.

0205 PM     TSTM WND DMG     HIGHWOOD                42.20N 87.81W
09/07/2021                   LAKE               IL   TRAINED SPOTTER

            20 INCH DIAMETER TREE DOWNED ONTO ROAD.

0205 PM     TSTM WND GST     HIGHWOOD                42.20N 87.81W
09/07/2021  M56 MPH          LAKE               IL   TRAINED SPOTTER

            PEAK GUST. SMALL BRANCHES DOWNED.

0210 PM     MARINE TSTM WIND WILMETTE BUOY           42.14N 87.66W
09/07/2021  M54 MPH          LMZ740             IL   BUOY



0212 PM     HAIL             PAW PAW                 41.69N 88.98W
09/07/2021  M2.00 INCH       LEE                IL   PUBLIC



0218 PM     HAIL             4 WNW EARLVILLE         41.61N 89.00W
09/07/2021  E1.75 INCH       LA SALLE           IL   BROADCAST MEDIA

            TIME ESTIMATED FROM RADAR.

0225 PM     HAIL             EARLVILLE               41.59N 88.92W
09/07/2021  E2.00 INCH       LA SALLE           IL   PUBLIC

            MULTIPLE REPORTS OF HAIL UP TO 2 INCHES IN
            DIAMETER IN AND NEAR EARLVILLE. TIME
            ESTIMATED FROM RADAR.

0225 PM     HEAVY RAIN       1 NNW LIBERTYVILLE      42.30N 87.96W
09/07/2021  M1.32 INCH       LAKE               IL   EMERGENCY MNGR



0230 PM     MARINE TSTM WIND HARRISON-DEVER CRIB     41.92N 87.57W
09/07/2021  M42 MPH          LMZ741             IL   C-MAN STATION



0252 PM     TSTM WND DMG     4 SSW LELAND            41.56N 88.82W
09/07/2021                   LA SALLE           IL   TRAINED SPOTTER

            REPORT VIA NWS CHAT FROM SPOTTER OF 3 INCH
            DIAMETER TREE BRANCH DOWN NEAR LELAND, IL.

0254 PM     TSTM WND DMG     6 NE SENECA             41.37N 88.53W
09/07/2021                   GRUNDY             IL   TRAINED SPOTTER

            NUMEROUS TREES DOWNED AND ONE SNAPPED IN
            HALF ALONG US ROUTE 6 BETWEEN MORRIS AND
            SENECA. POWER POLE ALSO DAMAGED. TIME
            ESTIMATED FROM RADAR.

0255 PM     TSTM WND DMG     MORRIS                  41.36N 88.42W
09/07/2021                   GRUNDY             IL   TRAINED SPOTTER

            NUMEROUS TREES AND WIRES DOWNED IN AND
            AROUND MORRIS. TIME ESTIMATED FROM RADAR.

0255 PM     TSTM WND DMG     MORRIS                  41.36N 88.42W
09/07/2021                   GRUNDY             IL   TRAINED SPOTTER

            REPORT OF 16 INCH DIAMETER TREE SNAPPED.
            TIME ESTIMATED BY RADAR. LOCATION IS
            APPROXIMATE.

0255 PM     TSTM WND GST     3 E LISBON              41.47N 88.42W
09/07/2021  M72 MPH          KENDALL            IL   TRAINED SPOTTER



0255 PM     TSTM WND GST     5 SE LISBON             41.43N 88.42W
09/07/2021  M61 MPH          GRUNDY             IL   ASOS

            WIND GUST OF 61 MPH (53 KTS) REPORTED BY C09
            ASOS.

0259 PM     TSTM WND DMG     3 WSW PLATTVILLE        41.51N 88.43W
09/07/2021                   KENDALL            IL   TRAINED SPOTTER

            REPORT OF TREE BRANCHES AND LIMBS DOWNED BY
            WINDS ESTIMATED TO BE 60 TO 65 MPH.

0302 PM     TSTM WND GST     MINOOKA                 41.46N 88.26W
09/07/2021  E85 MPH          GRUNDY             IL   NWS STORM SURVEY

            AN NWS STORM DAMAGE SURVEY TEAM FOUND
            WIDESPREAD TREE DAMAGE, INCLUDING NUMEROUS
            DOWNED, ACROSS MUCH OF AND NEAR MINOOKA.
            SOME OF THESE TREES WERE DOWNED ON
            STRUCTURES. THE DAMAGE WAS CONSISTENT WITH
            THE HIGHEST WIND GUSTS TO AROUND 85 MPH.

0302 PM     TSTM WND DMG     MINOOKA                 41.46N 88.26W
09/07/2021                   GRUNDY             IL   STORM CHASER

            PICTURES OF TREE BRANCHES DOWNED ON A HOUSE
            VIA SOCIAL MEDIA. TIME ESTIMATED BY RADAR.

0307 PM     TSTM WND DMG     1 WSW MINOOKA           41.45N 88.28W
09/07/2021                   GRUNDY             IL   STORM CHASER

            REPORT VIA SOCIAL MEDIA OF TRACTOR TRAILERS
            OVERTURNED AT A COLD STORAGE FACILITY. TIME
            IS ESTIMATED FROM RADAR.

0308 PM     TSTM WND DMG     1 NE CHANNAHON          41.45N 88.21W
09/07/2021                   WILL               IL   NWS EMPLOYEE

            SEVERAL LARGE TREE LIMBS SNAPPED IN
            CHANNAHON. TIME ESTIMATED FROM RADAR.

0310 PM     TSTM WND GST     CHANNAHON               41.43N 88.23W
09/07/2021  E85 MPH          WILL               IL   NWS STORM SURVEY

            AN NWS STORM DAMAGE SURVEY TEAM FOUND
            WIDESPREAD TREE DAMAGE, INCLUDING NUMEROUS
            DOWNED, ACROSS MUCH OF AND NEAR CHANNAHON.
            SOME OF THESE TREES WERE DOWNED ON
            STRUCTURES. THE DAMAGE WAS CONSISTENT WITH
            THE HIGHEST WIND GUSTS TO AROUND 85 MPH.

0310 PM     TSTM WND DMG     CHANNAHON               41.43N 88.22W
09/07/2021                   WILL               IL   EMERGENCY MNGR

            CHANNAHON VILLAGE OFFICIAL REPORTS
            WIDESPREAD DAMAGE TO TREES AND POWERLINES.
            ESTIMATE OF 100 OR MORE TREES DAMAGED ACROSS
            THE VILLAGE WITH SOME TREES UPROOTED. A
            DOZEN OR MORE TREES ON HOUSES. SIDING DAMAGE
            TO AT LEAST 2 HOUSES. WORST DAMAGE APPEARS
            TO BE NEAR THE DUPAGE RIVER SOUTHWEST OF THE
            JUNIOR HIGH SCHOOL. TIME ESTIMATED BY RADAR.

0310 PM     TSTM WND DMG     2 ENE CHANNAHON         41.44N 88.20W
09/07/2021                   WILL               IL   LAW ENFORCEMENT

            CORRECTS TIME OF PREVIOUS REPORT FROM 2 ENE
            CHANNAHON. REPORT OF A TREE DOWN ON SB I-55
            AT US 6 AND OVERTURNED SEMI TRUCK ON I-55 AT
            ARSENAL.

0310 PM     TSTM WND GST     2 NNW MAZON             41.27N 88.45W
09/07/2021  M52 MPH          GRUNDY             IL   PUBLIC

            PRIVATE WEATHER STATION KILMAZONIO REPORTED
            A WIND GUST OF 52 MPH AT 3:10 PM CDT.

0315 PM     TSTM WND DMG     MINOOKA                 41.46N 88.26W
09/07/2021                   GRUNDY             IL   TRAINED SPOTTER

            SPOTTER REPORTS TREE BRANCHES DOWN. WINDS
            WERE ESTIMATED TO BE 60 MPH.

0318 PM     TSTM WND DMG     3 E ELWOOD              41.40N 88.06W
09/07/2021                   WILL               IL   TRAINED SPOTTER

            DAMAGE TO BUILDINGS AT A FARM ON THE CORNER
            OF W HOFF RD AND ROWELL AVE. ALSO TREES
            UPROOTED AND CORN FLATTENED. TIME ESTIMATED
            FROM RADAR.

0321 PM     TSTM WND GST     ELWOOD                  41.40N 88.11W
09/07/2021  E60 MPH          WILL               IL   AMATEUR RADIO

            REPORT OF ESTIMATED WIND GUSTS OF 50 TO 60
            MPH WITH SOME TREE LIMBS DOWN. TREE LIMBS
            ARE ESTIMATED TO BE 2 INCHES IN DIAMETER.

0324 PM     TSTM WND DMG     1 WNW MANHATTAN         41.43N 88.00W
09/07/2021                   WILL               IL   STORM CHASER

            PICTURE OF UTILITY POLLS SNAPPED AT THE BASE
            RELAYED VIA SOCIAL MEDIA. TIME IS ESTIMATED
            FROM RADAR.

0340 PM     TSTM WND DMG     3 E PEOTONE             41.34N 87.73W
09/07/2021                   WILL               IL   FIRE DEPT/RESCUE

            10 TO 12 POWER POLES DOWNED.

0342 PM     TSTM WND GST     5 NW GRANT PARK         41.30N 87.70W
09/07/2021  E60 MPH          KANKAKEE           IL   TRAINED SPOTTER

            TIME ESTIMATED FROM RADAR.

0342 PM     TSTM WND DMG     PEOTONE                 41.34N 87.79W
09/07/2021                   WILL               IL   TRAINED SPOTTER

            8 INCH DIAMETER TREE LIMBS DOWNED. TIME
            ESTIMATED FROM RADAR.

0345 PM     TSTM WND DMG     1 WSW KANKAKEE          41.11N 87.88W
09/07/2021                   KANKAKEE           IL   EMERGENCY MNGR

            REPORT OF A 6 TO 12 INCH DIAMETER TREE
            BRANCH DOWN.

0401 PM     LIGHTNING        2 ENE LIMESTONE         41.15N 87.93W
09/07/2021                   KANKAKEE           IL   TRAINED SPOTTER

            SPOTTER REPORTS LIGHTNING STRUCK A HOME
            UNDER CONSTRUCTION AND CAUSED ROOF DAMAGE.

0405 PM     HAIL             REDDICK                 41.10N 88.25W
09/07/2021  E0.70 INCH       LIVINGSTON         IL   TRAINED SPOTTER

            SPOTTER REPORTS DIME SIZE HAIL WHILE DRIVING
            NEAR REDDICK ALONG ROUTE 17.

0418 PM     TSTM WND DMG     2 WSW FLANAGAN          40.87N 88.90W
09/07/2021                   LIVINGSTON         IL   BROADCAST MEDIA

            LARGE METAL OUTBUILDING COLLAPSED. RELAYED
            BY WMBD. TIME ESTIMATED FROM RADAR.

0420 PM     TSTM WND DMG     FLANAGAN                40.88N 88.86W
09/07/2021                   LIVINGSTON         IL   PUBLIC

            LARGE TREE LIMBS DOWNED IN FLANAGAN. RELAYED
            BY WMBD. TIME ESTIMATED FROM RADAR.

0448 PM     TSTM WND GST     3 SW BAILEYS CORNER     41.05N 87.05W
09/07/2021  M68 MPH          JASPER             IN   TRAINED SPOTTER

            SPOTTER MEASURED WIND GUST OF 68.2 MPH AT
            INTERSECTION OF SR 49 AND COUNTY ROAD 255N.

0512 PM     HEAVY RAIN       3 W IRWIN               41.05N 88.04W
09/07/2021  M2.51 INCH       KANKAKEE           IL   COCORAHS

            THE 2.51 INCHES OCCURRED IN ABOUT 1.75 HOURS
            ENDING AT 5:12 PM CDT.

0530 PM     TSTM WND GST     4 WNW GIBSON CITY       40.48N 88.45W
09/07/2021  M54 MPH          FORD               IL   MESONET

            PRIVATE WEATHER STATION REPORTS 54 MPH JUST
            WEST OF GIBSON CITY.

0535 PM     TSTM WND DMG     GIBSON CITY             40.46N 88.38W
09/07/2021                   FORD               IL   BROADCAST MEDIA

            WIDESPREAD TREE AND POWER LINE DAMAGE,
            HOSPITAL ROOF DAMAGED, AND BASEBALL DUGOUT
            DESTROYED IN GIBSON CITY.

0555 PM     HEAVY RAIN       1 SSE GIBSON CITY       40.46N 88.37W
09/07/2021  M1.50 INCH       FORD               IL   BROADCAST MEDIA

            CORRECTS TIME OF PREVIOUS HEAVY RAIN REPORT
            FROM 1 SSE GIBSON CITY. 1.5 INCHES OF RAIN
            FELL IN ABOUT 45 MINUTES IN GIBSON CITY.
            TIME IS ESTIMATED BY RADAR.

0558 PM     TSTM WND DMG     GARDNER                 41.19N 88.31W
09/07/2021                   GRUNDY             IL   911 CALL CENTER

            REPORTS OF TREES DOWN ON POWER LINES AND
            POWER LINES DOWN ON ROADWAYS.

0601 PM     HAIL             3 SW ESSEX              41.16N 88.23W
09/07/2021  E1.00 INCH       KANKAKEE           IL   TRAINED SPOTTER

            TIME ESTIMATED FROM RADAR.

Minooka and Channahon Damage

Public Information Statement
National Weather Service Chicago IL
450 PM CDT Wed Sep 8 2021 /550 PM EDT Wed Sep 8 2021/

...NWS Damage Survey for 09/07/2021 Thunderstorm Wind Event...

.Overview...Thunderstorms produced widespread damaging straight
line winds across southern Kendall, northern Grundy, and eastward
into western Will County. This includes the communities of Lisbon,
Minooka, and Channahon. Much of the damage was consistent with
winds of 60 to 70 mph, however, there were pockets of more intense
damage consistent with 70 to locally greater than 80 mph winds.
Numerous trees were downed, including some into homes, and there
was also some mostly minor structural damage. Extensive ground
surveying did not reveal any conclusive evidence of tornadic
damage, though we will continue to evaluate any additional
information that becomes available.

&&

NOTE:
The information in this statement is preliminary and subject to
change pending final review of the event and publication in NWS
Storm Data.

Photos

Environment

On September 7, 2021, bands of severe thunderstorms raced through northern Illinois and northwestern Indiana. The thunderstorms were supercells, or storms with rotating updrafts. Supercells are known for producing all hazards of severe weather (damaging hail, winds, and tornadoes), and develop in environments with strong wind shear.

Map

MRMS Radar Reflectivity at Lowest Altitude (RALA) from 11:00 AM to 9:00 PM CDT, September 7, 2021

Let's look through thermodynamic and kinematic variables meteorologists with the National Weather Services often use when assessing the potential for severe weather.

1). Forcing, or sources of lift


RAP-analyzed 300 mb analysis from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021. 300 mb heights are contoured in black, wind speeds are shaded and displayed in wind barbs, and upper-level divergence is contoured in magenta.	RAP-analyzed surface analysis from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021. Surface pressure is contoured in black, temperature contoured in red, and dew points shaded and contoured in light blue.

The main "forcing" or sources of lift for thunderstorms on September 7 were 1). an approaching upper-level trough and associated jet max, and 2). a surface cold front. Both acted in tandem to lift low-level air to the point where it became freely buoyant.

2). Storm Mode


RAP-analyzed MLCAPE* (J/kg) from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021.*	RAP-analyzed Effective Bulk Shear* (kt) from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021.*

As the upper-level trough and cold front moved southeast into Illinois, they met increasingly unstable air as analyzed by mixed-layer CAPE (MLCAPE) of 2000 to 3000 J/kg. Such values typically lead to vigorous thunderstorms with strong updrafts. Additionally, the incoming upper-level jet max afforded appreciable deep-layer shear, both with respect to speed and direction. Effective Bulk Shear exceeded 50 kt across much of the area, which is more than sufficient for organized thunderstorms including supercells.

The strong "linear" forcing along the cold front as well as broad synoptic-scale lift afforded by the approaching upper-level trough was supportive of a long line of thunderstorms. However, the deep-layer shear had a large component orthogonal to the cold front, which promotes a semi-discrete "broken" band of thunderstorms. Taken together, the thermal and kinematic environment was supportive of intense, semi-discrete supercells as the dominant storm mode.

3). Severe Weather Hazards


RAP-analyzed Max 2 to 6 km Lapse Rate* (C/km) from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021.*	RAP-analyzed DCAPE* (J/kg) from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021.*

When assessing the threat of hail, we typically look at instability and lapse rates. Instability provides an approximation for the potential intensity of an updraft. Aside from looking at traditional measures of instability (e.g. MLCAPE), we often look at the rate at which temperatures increase or decrease with height, which is called the lapse rate. The mid-level lapse rate on September 7 was 8 to 8.5 C/km (meaning the temperature dropped about 8 decrees Celsius every kilometer), values that are considered very "steep" or indicative of considerable instability. Mid-level lapse rates greater than 8 C/km can support updrafts that produce hail larger than 2 inches in diameter, provided the other ingredients for thunderstorms are available.

The environment also had a large amount of Downdraft CAPE (DCAPE), with values analyzed greater than 1000 J/kg. Values greater than 800 are typically favorable for damaging winds, with the threat (and intensity of) greatly increasing as values exceed 1000 J/kg. Taken together, with a supercell storm mode, both large hail greater than 2" in diameter as well as significantly damaging winds were possible in the environment.

4). Composite Parameters


RAP-analyzed Significant Hail Parameter* from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021.*	RAP-analyzed Supercell Parameter* from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021.*

One way to visualize the necessary ingredients for severe weather is to use composite parameters that use calibrated instability and shear variables to make a non-dimensional parameter. We often look at the Significant Hail Parameter, where values greater than 1.5 are typically associated with the potential for thunderstorms to produce hail larger than 2.5 inches in diameter. We also use the Supercell Composite Parameter to assess the threat for supercells. Both parameters demonstrated that the environment was favorable for large hail and organized thunderstorms.

(Want to learn more about these parameters, or see them in real time? Visit the Storm Prediction Center Mesoscale Analysis page!)

5). Put it all together...

Lo and behold, supercells with damaging winds and large hail developed across northern Illinois and northwestern Indiana. One storm was particularly intense and produced accumulating, large hail over the town of Earlville.

Map

Additional Info

NWS Milwaukee Event Summary (Southern Wisconsin)
Northwest Illinois (NWS Quad Cities)
Central Illinois (NWS Central Illinois)
Northern Indiana (NWS Northern Indiana)
Central Indiana (NWS Indianapolis)
NWS Chicago Past Events Page

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Please acknowledge the NWS as the source of any news information accessed from this site.

Additional recaps can be found on the NWS Chicago Past Events Page